Air conditioning compressors are powered by an engine belt driven rotor or pulley, and it is important to be able to selectively power the compressor so that it runs more efficiently, in response to cooling demand, and not just in response to the belt. This is generally done through the use of an electromagnetically actuated clutch in which a ferrous armature secured to the compressor drive shaft, which is normally spaced axially away from the pulley, is pulled by a magnetic coil into abutment with a friction disk of the pulley. Some mechanism is needed to return the armature to its off position, and this is typically a simple spring or springs. A good example may be seen in U.S. Pat. No. 4,860,867 to Nishimura. Three separate springs, individual beam members, are bent when the armature is pulled in, and pulled in back when the coil is de-energized. Another, newer design eliminates the separate, metal springs in favor of a unitary plastic or elastomer disk. One such design, such as that shown in coassigned U.S. Pat. No. 3,348,213 to Bernard et al, used a fairly thick elastomer pad or disk, which provides a good deal of torsional cushioning and vibration absorption, in addition to return force. Another integral design uses a thinner disk of plastic, which are more compact, but significantly stiffer than elastomer pads. An example may be seen in U.S. Pat. No. 5,150,779 to Booth. A problem any plastic disk design faces is durability, especially in terms of fatigue and creep over the life of the disk.
All the designs described above create the armature return force basically through a continuous, simple, linear spring action. That is, as the armature is pulled in axially farther in, the spring is stretched proportionally farther. This creates more return force, but, in some cases, more return force than is really needed. The other aspect of armature return force is that it is the force that the coil must overcome to keep the armature and pulley engaged, and too great a return force may cause slippage. This may become especially significant as the armature wears over the life of the clutch. The growing gap between armature and disk means higher spring stretch, more return force, and higher potential for slippage.